Inelastic Neutron Scattering Spectra from Lanthanum Dihydride and Lanthanum Trihydride

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Abstract
The optical hydrogen vibrations in lanthanum dihydride and lanthanum trihydride have been investigated by the energy‐gain scattering of cold neutrons. The approximate frequency distributions, derived from the observed time‐of‐flight spectra, show peaks which correlate well with the known crystal structures. In fcc lanthanum trihydride the optical bands are peaked at 940 and 515 cm−1 corresponding to hydrogen vibrating at tetrahedral and octahedral sites, respectively, and in lanthanum dihydride there is a single peak at 825 cm−1 due to tetrahedral hydrogen. Other weak peaks have been observed and are thought to be due to vibrations of hydrogen atoms dissolved in the metal phase; this phase was identified by x rays in both samples. A large width of the optical levels, a common feature in all metallic hydrides, was also observed for LaH2 and LaH3.